The present invention relates to hydraulic fluid power steering systems and more particularly relates to such systems having an accumulator for supplying fluid in the event that the vehicle engine or primary fluid pressure source fails.
Industrial vehicles such as skidders, loaders and scrapers, or the like, are provided with hydraulic fluid power steering systems which make it possible for these large vehicles to be maneuvered with relative ease during normal operation. However, these vehicles become very difficult to maneuver once steering fluid pressure is lost. In order to diminish the safety hazard occasioned by the loss of power steering, it is a common practice, often required by law, for manufacturers to provide steering systems having some means for providing a reserve of steering fluid sufficient for an operator to maintain steering control at least until the vehicle is brought to a safe stop.
Heretofore, the most common way for providing a reserve of steering fluid for emergency steering has been to incorporate an accumulator into the main steering system. These accumulators have largely been of the bladder type and have usually been incorporated in such a way that their bladders expand and contract (cycle) during normal steering system operation in response to system pressure fluctuations. Such constant cycling significantly diminishes the useful life of the accumulators.
Also, it is a common practice to provide these systems with a pressure responsive indicator for apprising an operator of the fact that the steering fluid pressure supply pump is no longer providing adequate steering fluid pressure and that he should therefore take steps to bring the vehicle to a safe stop before the limited supply of emergency steering fluid is exhausted. Because the pressure of the fluid normally maintained in the accumulator is the standby pressure of the pressure supply pump and the pressure responsive indicator is set to respond at a value less than the lower limit of the expected system pressure fluctuations, the fluid pressure contained in the accumulator will be reduced from standby pressure to the indicator actuation pressure before the operator is aware that he is drawing on the reserve of fluid contained in the accumulator. Therefore, this amount of fluid is not available to the operator for use in bringing the vehicle to a safe stop and accordingly necessitates larger accumulators to be used than would be the case if their total reserve of fluid were available at the time the operator became aware that he had a loss of primary power steering fluid pressure.
One prior art steering system which appears to overcome the aforementioned problems attendant with the usual way of incorporating accumulators into steering systems is disclosed in U.S. Pat. No. 3,882,952 issued to Crabb on May 13, 1975. Specifically, the Crabb patent discloses an open center steering system incorporating an accumulator such that charging the accumulator with fluid and discharging fluid from the accumulator is controlled by an emergency steering valve such that the full charge of the accumulator is available for emergency steering. The emergency steering valve is "armed" automatically in response to a reduction in system pressure but is required to be shifted by means of a steering linkage in order to effect discharge of fluid from the accumulator for use in emergency steering operation.